When a change of variable occurs in a system, the system responds. In the case of positive feedback the response of the system is to change that variable even more in the same direction. For a simple example, imagine an ecosystem with only one species and an unlimited amount of food. The population will grow at a rate proportional to the current population, which leads to positive feedback. This has a de-stabilizing effect, so left unchecked, does not result in homeostasis. In some cases (if not controlled by negative feedback), a positive feedback loop can run out of control, and can result in the collapse of the system. This is called vicious circle, or in Latin circulus vitiosus.

Positive and negative do not mean or imply desirability. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to speed it up.
Positive feedback is used in certain situations where rapid change is desirable.

One common example of positive feedback is the network effect, where more people are encouraged to join a network the larger that network becomes. The result is that the network grows more and more quickly over time.

Audio feedback is a common example of positive feedback. It is the familiar squeal that results when sound from loudspeakers enters a poorly placed microphone and gets amplified, and as a result the sound gets louder and louder.

Addictions are another example of this type of feedback. The person engages in a loop where addictive behavior leads to more addictive behavior. Also, in social interaction, the cycles of human action and reaction in heated discussions could be seen as positive feedback.